This invention relates to a new direct-steamed laboratory minidigester and a method of operating it. The unit may be used to predict the operation of direct-steamed commercial-size digesters.
Commercial digesters in the pulp industry range in size up to 7200 cubic feet or larger. (Digestion is the process of treating wood with chemicals to separate the valuable cellulose from the other constituents of the wood, e.g., lignin). For obvious reasons, it is desirable that the values of various parameters relating to the commercial operation, such as, the required chemical charge and liquor to wood ratio, the yield, and the Kappa number, be known before commencing operation.
Typically, for this purpose, laboratory runs are made with a sample of the wood to be commercially treated. The pilot digesters employed are often from 30 to 50 gallons in capacity and are direct-steamed. Although runs in such equipment are time-consuming and require approximately 40 pounds of wood and relatively large amounts of steam and chemicals, these units are employed because they satisfactorily simulate commercial units.
Much smaller units (holding out fractions of a pound of wood) are also used, but they cannot accurately simulate a commercial direct-steamed digester because they are closed bombs and cannot be direct-steamed. (Direct-steaming provided both heat and agitation.)
In an effort to overcome the lack of agitation in these small units, they are sometimes rocked during operation by shaking devices. However, this also prevents accurate simulation of commercial direct-steamed digesters.
A laboratory digestion unit disclosed in U.S. Pat. No. 3,781,188 comprises several vessels of between 50 and 1,000 cubic centimeter capacity. The vessels are arranged in parallel so that the same digestive liquor may be circulated through each. If operated in this fashion, the same wood must be used in each vessel during a run. Valves are provided to allow isolation of each vessel at different times in the run. These units are not direct-steamed and, thus, do not simulate direct-steamed commercial units. These units may also be run as individual closed bombs, each, if desired, with a different liquor or wood; however, this too fails to simulate direct-steamed commercial operation.